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The carbon cycle: a simple explanation

The terrestrial biosphere's role in the carbon cycle


Carbon dioxide from the atmosphere is utilised by plants by photosynthesis. The carbon they absorb is allocated within the plant to make up its roots, wood and leaves. Some of this carbon is then lost - either when the leaves drop, or when the plant dies - and becomes soil carbon. Microbes within the soil breakdown this carbon and release it back to the atmosphere as respiration, in the form of carbon dioxide. This is the terrestrial carbon cycle on a small scale (i.e. on the scale of individual plants).

On a larger scale (i.e. across geographical regions), the distribution of vegetation is important in the carbon cycle. Different plant types store different amounts of carbon, but they grow at different speeds and favour different conditions. For example trees can store more carbon than grass (per unit area of land covered), but they take a lot longer to grow. So if a previously barren area of land becomes fertile for some reason then grasses will grow first, but trees may take over later. The local climatic conditions, and how they change over time, determine which type of plant dominates in any given location.

Human activity also changes the land use, and hence the carbon stored by the biosphere - cutting down trees removes a potentially large absorber of carbon dioxide and if the wood is burnt, or left to decay, then the carbon is released back to the atmosphere. Disturbance of vegetation also affects the soil - deforestation can also lead to large amounts of carbon being lost from the soil. This has an impact on the fertility of the ground and may affect future vegetation growth in the area. Such changes in land use (predominantly in the tropical forests) accounted for the most significant part of anthropogenic carbon dioxide release during the 19th Century. It was not until about 1950 that fossil fuel emissions became significantly larger than the source from land use change. Present day emissions due to anthropogenic land use change still amount to around 1 GtC per year.